This invention concerns rotating machines, such as turbines and pumps, which are susceptible to catastrophic failure in operation. Such failure may be due to shaft cracking or cracking of a component (such as a blade or rotor) attached to the shaft. The invention concerns means for noninvasively monitoring such machinery to anticipate occurrence of such failure, so that the machinery can be shut down before the failure occurs.
Attempts have been made in the past to detect cracks in pump shafts and turbine blades, but the effectiveness of the methods attempted has not been established. Moreover, these methods are considered impractical for wide application. A method investigated by Pratt & Whitney involved detecting passage of individual turbine blades by their interruption of a light beam, synchronizing blade signals with an external reference, and monitoring the difference between actual and expected blade rotation angles. Hardware proposed to implement this idea included fiber optic probes to be installed inside the turbine to transmit and receive light beams, and related electronic apparatus. This system's requirement of installing optic sensors deep within a turbine and then routing signals out of that environment posed serious implementation difficulties because of the harsh, live-steam environment in which the apparatus had to be placed.
Another method, under investigation by Liberty Technology Center, involves installing Doppler instrumentation within a turbine. This method poses similar implementation difficulties because of the same harsh-environment problem.
Noninvasive methods have been proposed, also, based on use of externally placed vibration sensors. However, the multitude of vibration frequencies present in such signals has been a major obstacle to extracting signals representative of blade failure conditions.